Accidentally on Purpose
by ItsaRandomUsername
Summary: As autumn meanders on, the denizens of Fuyuki City become impulsive. Such events lead to a small mishap shared between Illyasviel von Einzbern and Emiya Shirou.


_**Disclaimer:** All participants engaging in the various acts of coitus depicted here are adults of responsible, legally consensual age. I do not own Fate/stay night or anything related to it. This story was written solely for entertainment purposes, not for monetary profit. If anything, consider this free advertising._

* * *

 _ **Accidentally on Purpose**_

It was an awkward time of year. Autumn had come, but had still not yet established itself in the eyes of the world. Though the sea breeze was finally chill enough to raise goosebumps on the back of necks the sun was still bright and hot. The Indian summer that had dug its roots into Fuyuki City had finally begun to wane, but the memories of the true summer's heat still lingered in the people's hearts. Gentle mania was the name of the day—and the people wanted to indulge in a little bit of summer for a little bit longer however possible, with whatever sprung to mind.

These weather conditions were directly responsible for why Shirou was in his storehouse. Why he decided to toil away so attentively on this latest project. Why he was so determined, so focused on restoring it that even as he wrapped up his progress on it for the day he didn't notice the sneaky pitter-patter of sneakered steps that told of someone's arrival into his not-so private sanctum.

"Gotcha!" a certain someone pounced upon him from behind and covered his eyes with her hands said. "Now then, can you work on what you can't see?"

"It's been a couple of days. What brings you here, Illya?" Shirou asked, his eyes still very covered by the small pair of hands. He even squatted a bit, so that she didn't have to strain herself so much to reach.

"I came," she said. "for _you._ "

"The sun's still out. It's a little early for a midnight tryst," he said back.

"And it's too early for you to act out as the straight man to me!"

"Then don't set me up for such an obvious joke. Especially now, when low-hanging fruit has good appeal." Shirou gave a small sigh as he momentarily lost himself in thought, with regards for his current fixer-upper.

Illya's curiosity was piqued enough that she allowed herself to peel away from him and get a better look at the project. "What is that thing anyway, Shirou?"

"It's like this, Illya," said Shirou as he stood at the side of his current project. He ran a hand through his hair, and a slight sweat on his brow glistened dully in the shade of the building. "You know how when cats like their owners they bring them dead animals?"

"To attribute feelings of 'like' to cats is to be far too kind to their lot." Illya remained unimpressed by feline generosity. "Those're bribes if there ever were any."

"…anyway. Some people say it's because since cats don't ever see us hunt for our own food they're convinced we don't know how to. Since we don't have predatory instincts, and that maybe bothers them, they bring it to us, just to show they have us in mind."

"Who can best serve them when it is convenient for them, you mean."

Shirou subconsciously glanced over at his pile of random bits and bobs of a kettle and pot conveniently right next to each other. He cleared his throat but once, and leaned over to place a hand on top of the thing. "Gifts, bribes, care packages, or otherwise, it stands to reason that the bigger the cat, the bigger the prey. Therefore—"

 _"—you can't be a much bigger cat than a tiger."_ Illya finished that line of thought for him. "So then, Taiga dropped off this…?"

"Soft serve ice cream machine, yep. I can take a hint when she's been as obvious about it as she has." Shirou gave the relevant device a pair of stout, empathetic pats. "If I get this gizmo ready before the heat goes completely we could all treat ourselves before it gets put away until next summer."

Or when he'd regift it back to Taiga. One less thing cluttering up his workspace and she gets to eat ice cream whenever she wants. Normally, that would have been a win-win, but in this case…

"Hm? Really?" Illya's eyes light up. "Could we try it out right now?"

"About that—" Shirou said. "Let me show you." He nodded her over, and Illya took that as her cue to place her hand on his, her fingertips brushing the back of his hand as she situated her palm.

They saw the same sight through the same eyes. In that moment there was no Illya, no Shirou. There was only "I," and "I" saw the very framework of the machine.

"It's more than just the nozzle, huh," with deeper understanding than before, Illya said this. As if that was what broke the spell, her words grounded the pair back in reality, and a vague feeling of loneliness lingered in their minds for a fleeing instant as they separated. "A few of the grinders are corroded, as well. Those are parts you must order."

"I called the distributor and they're in the mail," Shirou gave a shrug, and the amount of work and time on such short notice now was reason enough for his small moment of exasperation.

"She's been difficult about the wait, hasn't she?"

"Pretty difficult for a big sister," replied Shirou. "Although _that's_ nothing new."

Despite the meaningful look he gave her, Illya only gave a knowing nod of solidarity in return. "Verily," she said, her eyes closed.

"Okay! I can't do much with this thing right now, so I'll give it a rest." Shirou spoke as he left the shed and stepped into the afternoon autumnal sun. "I'm gonna go get cleaned up… and then we can hang out some more."

"Very well, then. I can wait. See you in a bit, Onii-chan?" standing alone in the workshop and a tranquil simper, Illya peacefully acceded.

"Uh, yeah, okay…" he replied.

A moment later, he was out of sight, and she spoke aloud to herself. "Well, it should be fine if it's not quite ready yet! After all, things are at their best when they're a little bit underdone!" said Illya.

She then made a beeline to the kitchen to grab the cream, sugar, and vanilla.

 **.**

 **\- ] | [ -**

 **.**

He shut the bathroom door, but had not locked it. His shirt was off and stowed away in the cubby hole, soon about to be joined by his trousers by the time he heard the crash, the scream.

It was full of surprise. Fear. _Pain._

 _ **—Illya.**_

Shirou's heart stopped and his legs moved on their own. His breath was stolen, and that span of a breath taken was all it took to reinforce his full self. His mind was clear of nearly all thought as pure, mechanical instinct jetted his body at meters per second. The Holy Grail War and the decisions made as part of it had forged him in fire. These days he was strong.

Illya, though—

Shirou couldn't think about that. Something was in his way. Separated him from her. So Shirou dove through that mere obstacle, heedless of the shards of broken and breaking window that clattered against his bolstered being, crunched in the grass underfoot. Lowly glass was nothing against his iron will.

Whatever. He'd pay the Old Man to get it replaced. That guy, Fujimura, he'd understand why Shirou did it, though Shirou would still be obliged to do his part in the reparations. Not that Shirou had thought about any of that in that moment. As he blazed through the yard, there was nothing else, no one else in his mind but—

"ILLYA!?" Shirou's voiced quirked as he blew into the shed and called out to her, desperate for a response.

She instantly responded, her voice, too, wracked with pain. "Shirou–! I really messed up–!"

"Shit!" he said.

It broke his heart, was one of the few things that could utterly rip through his armor, grab his fragile heart and soul by the balls in a deathgrip and _squeeze_ until it popped like a fireworks and his world reduced to a dark mental landscape of anguish and pity.

All because he'd _let_ it affect him, and wouldn't have that any other way. That was their price for survival, each of them paying it in their own way.

Knickknacks were chaotically strewn across the floor, the pile they had once been part of ruinously toppled over. So too did Illya lay there—as the very machine that Shirou had spent the morning on crushed her leg beneath its steel.

…and for some reason was splattered with a curious milky fluid.

"What the hell?" The sight made him anxious, gave him pause. But, only a pause. It could only keep his instincts—his own personal curse of Lorelei—at bay for a fraction of a moment. "I mean, I'm coming, Illya!" In one of Illya's pained, watery blinks he was at her side and pulled the heavy machine off of her, orienting it back to its original upright position. The same substance dribbled out from its leftward nozzle, and the feelings that the sight of her like this caused in him eased up.

"OKAY! It's off!"

"How is it?! Is it bad?!" Curiosity more than fear colored her tone of voice, even shaken up as she was by the seconds-ago accident.

"One sec." Shirou replied. Now that they were together for this they were far calmer than before. Right away they could tell her ankle and foot were swollen red, a bruise at the center of the damage a sickly blotch of widespread purple that stood out against her fair skin like blood in snow. He put his hand to it. Illya winced, hissed just under her breath.

There was only one way to tell. Shirou's hand was already there, physically. All he had to do was reach out.

The way his energy poured out of him into her felt like a vase of warm water had its contents slowly poured over her whole body, felt like it soaked her in a foreign, probing heat that bathed her inside out. He read her, everything about her physical self, and it made her feel so many things. Vulnerable. Tepid. Cared-for. Exposed. Secure. Good. For that irreplaceable wink of time Illya forgot about her injury and gave herself over to Shirou.

"Shirou…" she found comfort in his name, and would have repeated it now until the end of time as a mantra if it meant that peace would last.

Then it was done, and in that agonizing moment after it felt for her like all magic had disappeared from the world. There was slight sweat on Shirou's brow again. Human bodies were exponentially more complex to read than the devices they used and needed in their lives. But, Shirou was more than familiar enough with Illya's that such a process for him was no more than just a little taxing. By now, reading the brushstrokes that made up the painting of _her_ being was degrees away from being second nature to him. Perhaps in the future this would become an affinity to rival or even surpass his innate talents for matters regarding bladed weaponry. Perhaps.

For now, he was able to not have a migraine and diagnose her at the same time.

"You lucked out, Illya. It's a sprain. It's not broken, it's just painful."

"That is a relief," she sighed.

"So, what happened?" Shirou asked, taking the moment to now address the proverbial elephant in the room. "You're hurt. You're dirty." He took a finger and gently ran it across her cheek, catching some spilled drippage on the tip to then bring it to his lips. "You're— _tasty?"_

"O-Onii-chan! Of course I am!—and of course it is. I made it, so it must be so." Illya turned as red as her wound but took it as a compliment, and in stride.

"The question is why," said Shirou. "Why try to make—" He saw cream from an overturned bottle, vanilla partly spilled from a vial, and scattered sugar all over the floor. "—ice cream when you know for a fact that the machine's busted?"

"It was hardly not working. There was nothing to stop it from _making_ ice cream," she replied. "I just didn't expect it to blast me!"

"Not that I've trained you enough for you to reasonably make that call. You're too inexperienced of a mechanic." He retorted.

She merely shrugged and countered with "I just couldn't wait."

"—Ah." That made him think of the last summer, the passing fall, and impending winter. "Illya, what am I going to do with you?" he sighed.

"You should kiss it to make it feel better!" Illya suggested.

He did so.

"Ow!"

"Does it still hurt?"

"…Some."

"I'd be amazed if it didn't. Then I'd travel the world as a wandering miracle worker, healing them with my magic spit."

"You wouldn't leave me."

"Of course I wouldn't," Shirou stepped away to open a drawer. He drew a towel from within and laid it open where there was room. From the side of cabinet he took hold of a first-aid kit and likewise set it down. "Good thing Fuji-nee always keeps this thing well-stocked, just in case. …Now, c'mere." and he scooped her up in his arms.

Illya quietly whined under her breath, quiet enough that the pain was like a secret whispered between her and him. The pain came and went, and when it went she took advantage of the moment and asked "Hey, Shirou. There's ice cream on my face and blouse, and you haven't a shirt on—how do you think we look?"

"Dangerous." He answered as he laid her on the short-order makeshift cot, cracking into the box of medical supplies. "Does it still hurt?"

"Ouch." She affirmatively replied, with a gesture that was as much wink as wince.

"Right." He nodded. "Once I stabilize it so it doesn't hurt so much you can focus and take care of the rest."

"What if I like being pampered by you too much?"

"Just lift up, I mean, elevate the leg."

"Sure, sure." Taking care to not accidentally squeeze the swollen red injury, Illya pulled her knee to head, straightened her calf, and made it parallel to the floor. "Like this?"

"I can see your panties right now."

"Okay!" And Illya lifted her leg back further, her violet miniskirt hiking further.

"O-OKAY!" And Shirou went to digging through the first-aid in response. "There's bandages and gauze we can use to compress it," he said. "Forceps and scissors for the worst-cases, rubbing alcohol that might come in handy, and—"

He froze, gasped at what lay within.

"Shirou?"

"Uh, sorry. There's some painkillers in here, too. Morphine."

"Painkillers aren't weird to have in a first-aid kit, though?"

"No, no, not weird at all, nope."

""Then why are you acting so weird?" Illya rolled onto her side, putting her within reach of Shirou. She swiped the bottle from his hands to get a better look at it.

"Illya—!" Shirou protested.

"…Shirou…" Illya replied after looking at the bottle. "You said that Taiga is responsible for this first-aid kit?"

"…she's always took it upon herself, yes."

"So why is this morphine a suppository?!"

The conclusion obvious, their minds were full of poisonous thoughts straight from the gutter.

"Don't make me think about it any more than I have to…" Shirou said.

"Damn that genderbent Genji Hikaru wannabe, hoping that she'd net herself some Nee-san points if you got into an accident!" Illya furiously blushed with indignation and other thoughts. "She must have thought herself a clever beast, and she would have been one, too, were it not for me, here to foil her vixen plans."

"Illya…?"

"Shirou…" said Illya. "You want to heal me, right? You don't want me to be hurt anymore, right?"

"—Yes." Regardless of the tumult in his heart, obvious leading question or otherwise, he answered it the only way he possibly could.

"You're my medic now! There's no other way! To cure me, to absolve me of this pain—" Illya whipped out one of the hefty pills, the size of one of his thumbknuckles. "—you have to stick this in my butt!"

"Illya, a _doctor_ cures people! A _medic_ just makes them more comfortable as they d—"

"Ssh! I need this. Right now. Now. Ow. Owowow! See, it still hurts! See?!" With the bottle in one hand, Illya squeezed her swollen ankle. Fat tears welled up in her crimson eyes and spilled down her intensely rosy cheeks like the floodwaters from a pair of broken dams.

"Hey! Stop that! You'll make it worse!"

"I changed my mind! I don't need you to pamper me like you've been, just this much alone will do!"

"Illya, you really should—"

"You're mine. Mine alone. I shan't lose you to anyone else's tricks, okay? So please—"

Illya rolled onto her belly, got to her knees. Her cheek pressed into the soft towel, looked back at him with hardened, moist eyes as her hips raised her other cheeks into the air.

 _"—I need it, Shirou."_

"Well, it's been three times. I couldn't possibly refuse anymore." Especially when presented to him on a silver platter like this. "Alright, Illya." Shirou reached for the bottle. Her fingertips brushed his as Illya let him take it from her hand. "Just so you know," he gently warned, pill in one hand as he flipped her skirt up and began to roll her panties down her thighs with the other. Shirou spat on the pill, rubbed it between his forefingers until it was wet and shiny. "It'll probably be a little tight."

"You don't say?" Illya coyly giggled.

"Hm." Shirou quietly vocalized, the only rational response to make in the face of Illya's brand of chaos.

Her panties off, caught on her knees, her ass was free, and the air gave its svelte and subtle curves gooseflesh. But, that wasn't the sight that gave, again, Shirou pause.

That honor went solely to the white-as-her-hair topaz inlaid in the obsidian buttplug that gave him a winking peek-a-boo from in-between her cheeks.

He really should have known better. He had just scanned her, after all. But what he had scanned was her body. This foreign object had paradoxically flown under the radar precisely because it wasn't a natural part of her body.

"What the hell?" Again, rational vs. chaos. "Illya?"

"Yessssss?"

"What's this?"

"The reason I was gone these past two days." She wiggled her hips for emphasis.

"This was in here the whole time?"

"No," Illya replied. "I took it out many times, because I wanted to feel it slip back into my body. I wanted to feel it, Shirou. As I walked in the castle. Through the forest. Around town. My self-study. My _homework."_

The glimpses he'd caught before had been too brief to confirm anything beyond the realm of fantasy. That's what he'd rationalized to himself. It was a trick of the eye. Or spots caused by the heat. But no, it existed, had never been sweat—

—her panties had been this moist from the start.

"Homework, you say? Is there a test?"

"Mhm, although the proctor has only now finally gotten the memo, hasn't he?"

"All of this," Shirou gestured towards the mess and her messed-up foot. "was this the 'student's' ploy?"

"Hardly, Shirou." Illya's fingers danced across the surface of the towel. "Still, fate's a funny thing. Like this, it really is a pop exam, isn't it?" Again, her hips wiggled.

" 'Pop,' you say." Shirou took the hint. He grasped the crystal end and steadily pulled. Illya moaned under her breath, a sultry sound soft enough for him alone to hear, had they had the company of others in the storehouse. The rod glistened, having been lubed up again and again, and left a glassy rim in its wake.

There was no chance for him to resist, nor had he the willpower to do so.

Shirou brought his face to Illya's cleft and met it with his tongue. It tasted like nothing in particular. It was immaculate, pure as water. No, he decided after tasting further; it was subtle. Instead of water, rather, it was like spring water, tasting crisp and of natural minerals—true wild spring water. It lingered cloyingly, refreshingly, addictively on the tongue.

"Shirouuu, how is it?" Illya's voice rose higher as he tasted her.

"It's tasty." Shirou pulled away from Illya's taut rump to curtly answer, his voice honey to her ears.

"I told you so," she giggled back, and that giggle warped into a tickled moan at the return of Shirou's oral explorations.

Not only tasty, he decided, as he rolled his tongue around like the hand of a drunken clock, swirling around in her ass, lapping at and into the elastic entrance. It smelled clean, too—a scent as mild, as profound as fresh river valley wind blowing over the surface of slate cliffsides. Shirou moved up her backside to gave Illya's tailbone an appreciative kiss. "Wellll? Hh!—do I meet your standarrrrds? Huh?" Her knees came together, she balled the towel in her hands, and Illya gave a shiver of delight as she squeaked like a kitten.

"So far they exceed them." A strong part of him was reluctant to peel away from her pert, little peach bottom. A stronger part thirsted for that point of no return surely as strongly as she did. Coming down off of her ass-high, asked "Do you still need this, Illya?" as he held up the pill he had the whole time, still wet with his spit from before, and finally remembered after all this time.

"That is life-saving medicine. Of course I need it to get better, Onii-chan."

The lightly heady moisture that clung to her thighs and poured from within was certainly proof that if _someone_ didn't get _something_ then _someone_ probably would feel like dying.

"I'll see what I can do," he replied.

Wet from the toy and his tongue alike, he easily pressed the pill into her depths with his forefinger. Welcoming but cozy, it felt less like he was pushing and more like he was being sucked in.

When his knuckle met with her asterisk, Illya whined to Shirou "Make it go deeper! Finger alone isn't enough!" her voice awash with urgency and need as she reached back to stroke his arm.

He concurred. "Yeah. Wouldn't want it to fall out by accident." His finger retreated; her butt still retained its shape, elastically agape, as if it had been abandoned in its time of need. "You won't have to wait long." Shirou gave the asshole a sign of apology with one hand and undid his pants with the other. His tool stood on-call, the bell end almost polished with mirror sheen from all of the precum he'd been leaking.

It was—he was—111% more than ready for a insertion mission, a medical expedition.

"Do it, _Shirou."_ She commanded him, her ruby eyes so dilated that sunlight would surely hurt them.

He did so. Her hips meet with his. Illya's ass was just as accommodating, just as needful. He easily hilted into her, every inch of his pushing the pill into the inescapable depths of her lonely body, and every inch made her voice breathy and raised another octave of aching pleasure. "Ah, ow, ah!" Her sprain made her twitch in pained spasms, but she was too far gone to care about herself like that. "That's it, Shirouuu! It won't ever escape nowww~!"

Speaking of the suppository, or Shirou himself, that statement was pure gospel.

Illya continued to accept Shirou's thrusts, even as he hoisted her up, held her legs under an arm to keep them elevated and pain-free. Her panties had slipped further, hanging around her ankles, caught on the tennis shoes she still had on.

"Harder, Shirou-! Faster-!" Illya egged him on, words cracking and catching in her throat. "I want t-to feel it ahlll, b-before I numb from the inside-owut—!"

After all, that pill was a painkiller.

Reminded that it was a race against time to feel before she couldn't feel, Shirou used his free hand to hold her small body close to his. Hidden within the cloth folds of her soiled sleeveless blouse, Illya's erect nipples poked through the thin white material, her slim chest bouncing subtly in-tune with his own movements, up with his up, down his down. His thumb rested in her bellybutton as his core finger worked her blushing bead like it was the controller to her body itself. It seemed like every touch administered there made her leak more and more, every touch raised her voice higher and higher.

Shirou's pants were heavy and hot against her cheeks as ran his tongue across her face, almost like a mother cat cleaning its young, the image being appropriated for his and her pleasure, tasting what streaks of ice cream remained that hadn't been wiped away when she was facedown in the towel. Sugar-sweet vanilla danced on graced his tastebuds, and he invited her to taste her creation, his tongue and lips her dessert bowl. "Mhmmmnnn…!" Shirou and Illya moaned into each other's mouths, their emotions synching in full.

"SHIROooOUuUUuU!" She cried out, and her spring flowed over with him still captured by in her seductive grotto. He worked her harder with his hand to make the ecstasy last, and they were glad that the towel was there. "I'm in heaven, Shirou!"

"Illya! I'm so close!" Unable to do anything else but give in to the earthly delights, Shirou gasped helplessly.

"Yes, Shirou! I must have it! Every drop! Go—!"

—It was then Illya took the pill that she had been hiding in her palm the whole time and gave Shirou a reach-around. Her tiny finger slipped into his bumhole with ease, sending the not-inconsiderable suppository way up his gut. Her finger hooked, and it pressed his prostate just right, right when it mattered the most…

Yes, sisters, big, small, or some hybrid of the two, were truly difficult.

 **.**

 **\- ] | [ -**

 **.**

"You know, maybe now's a good time to remodel it into an outdoor bath," was suggested as the two of them walked through the backyard, back to the house, and they saw the broken bathroom window he had dived through.

Then the painkillers finally kicked in, and the rest of Illya and Shirou's afternoon was spent watching a perfectly mundane newsfeed, loopy on the medication and giggling like idiots the whole time at some inside joke they had discovered while in the middle of watching the program.


End file.
